Probably means nothing in your case but, on our first trip to Yellowstone, our Onan started, then quit, did it a couple times, while looking around, I noticed the altimeter setting was set at the lowest setting, in Yellowstone we were at 6000ft +/-, soon as I reset it to 5000+ it started & ran without a problem.

The only time my 4K Onan cranked but refused to start, it turned out to be the magneto. I discovered the bad mag by holding the plug against the Onan body (cooling fins IIRC) whilst it ..... no spark. A new mag assembly and it has run fine ever since. But then mine is 12 years old; has approx 500 hours on it and had lived down near the Calif beach for a several years before I bought it (the OEM mag turned out to be very corroded.)

Even though your Onan is almost new by comparison, I would VERIFY it has spark. If it passes the spark test, at least you know you probably need to concentrate on fuel delivery (and not just to the carb inlet....to the cylinder.)

New update. I drained the carb. Got it to crank up but was rough idle. Actually not rough as it was pulsating. It stalled out after 5 minutes and now stalls right out after starting. Going to try more carb cleaner and clean the fuel lines. Got code 15 and 36. Any input?

I assume it has spark because it starts, I would still pull a plug and see if the spark will jump at least 1/4 in to ground and is nice and bright to rule out ignition problems. I would then disconnect fuel line to carb remove air cleaner and spray gas or carb cleaner into carb and see if it stays running if so I would suspect a fuel delivery problem. I have had two intermittent problems that drove me crazy like that. One was just a bad 12v connection to the fuel pump, the other was the pick up tube going to the fuel tank. I am sure yours in not cracked or rotted but it can be loose do to someone not tightening the hose clamp. Also you can remove the jets and spray the holes. You manual will tell you how many turns to open each one as an initial setting. Being this new I would think it is something simple. Also try disconnecting the sensors to see if that helps. If you try these things and report back you will get better suggestions.

Sure sounds like a gummed up carb. You can't leave it for 60 days with gas in it and not expect problems. Two weeks is about it. Buy a new one or take the old one off and put it in a gallon of ChemDip carb cleaner overnight. It's a bit of a pain to get off but it can be done without taking it out.

Onan standard warranty on consumer products is 2 years or 2000 hrs, plus an additional third year on major components. fault code 36 is displayed when gen slows down or stops without a command from control board or stop switch (usually fuel related) code 15 is low frequency/speed.
Sounds like you have carb problems, will need cleaned or replaced. Of course, Cummins-Onan does not consider a gummed up/varnished carb to be a warrantable issue

I tend to agree on the carb. If any type of gasket needs to be replaced, make sure it gets installed the correct way. Sometimes is hard to tell, I learned the expensive way to the tune of $450 on a boat generator having the same symptoms. Had to call a factory tech who knew exactly what it was. Reversed the gasket.
It's funny, when I had genny problems while on the road, a repair shop offered me help as long as it wasn't an Onan. He said he refuses to work on them any more. He gladly worked on my Honda which I always carry as my backup genny and has become my primary unit to take care of my boondocking charging needs because it's more quiet. $35 and 45 minutes for the repair.
I do alternate running both genny's just to keep the fuel in circulation.

I had the same problem with my new MH back in 2006. Would start up and run for a minute and stall out. I would have to wait several hours and it would start again and stall again. It turned out being a carb float issue.